CHAPTER 2: LITERATURE REVIEW
2.15 Structural considerations
Supporting a green roof over a structure requires an effective structural support system.
An effective structural support system is much easier to implement in structures that are yet to be designed. This is based on the fact that the design of a green roof can be taken into consideration in the preliminary design of the structure whereas, in existing structures an analysis of practicality must be performed prior to the construction of a green roof.
Structurally, one must take into consideration the effects of dead loads, live loads and depending on the type of plants being planted on a green roof, wind loads upon the structure. Furthermore, all designs must meet the regulations proposed by local building codes and any relevant design codes. When considering a green roof system, the following are common load systems that one can expect (Scholz-Barth & Weiler, 2009):
2.15.1 Load effects
One of the main factors that forms an integral part of the determination of two of the most important considerations for green roofs, feasibility and cost, is additional loading (Kuhn
& Peck, 2008). For structures in the design stage, additional loading imposed due to the implementation of a green roof can be taken into account with relative ease. However, for an existing structure, the additional loading has to fall within the, governing, carrying capacity of the roof.
2.15.1.1 Dead loads
In terms of dead loads, one must consider the self-weight of the elements being used in the green roof. In a direct green roof system, the self-weight of the elements ranging from the waterproofing layers to the plants being planted in the system have to be taken into account. Furthermore, depending on whether the direct green roof system is classified as extensive or intensive, the weight of the soil layer will differ due to the depth stipulated by each layer. In contrast, in a modular green roof system the self-weight of the modules,
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together with the plants and any materials utilised as protection for the roof, need to be taken into consideration (Scholz-Barth & Weiler, 2009). Gartner (2008), states that in order to account for future additions of growth mediums to the green roof system the specific depth should be increased by 15%. In addition, the researcher provides questions that aim to assist in the determination of Dead loads. These are:
1. What type of green roof system is to be designed i.e. direct or modular?
2. Is the green roof to be sloped?
3. What will be the depth of the green roof system?
4. What plant life is to be utilised in the green roof?
5. Will the green roof consist of trees or other taller plant life?
6. Will there be decorative pieces i.e. water features, boulders, etc.?
7. Will there be water storage or retention systems upon the roof?
2.15.1.2 Live loads
Live loads will vary depending on the accessibility of the roof upon which the green roof is to be situated. As such, in cases where a green roof is designed to be accessible to provide access to groups of people, the live load will be at a maximum. In cases where a green roof has been designed for access but only to provide maintenance, the live load will be at a minimum (Scholz-Barth & Weiler, 2009). According to Gartner (2008), live loads will depend and vary as per local codes and occupancy type. The researchers recommend that extensive green roof systems be designed for a minimum of approximately 60 Kg/m2 and intensive green roof systems to a minimum of approximately 100 Kg/m2 when considering live load reductions. In order to assist in determining Live Load, the following questions should be asked (Gartner, 2008):
1. Will the green roof be utilised by people or will it only be accessible during maintenance?
2. Will the green roof be accessible to vehicular traffic?
2.15.1.3 Wind loads
From a structural point of view, wind loading can be considered as negligible when acting on grass or low-lying plant life. However, it must be taken into account when green roofs contain trees and other taller plant life. With that said, Scholz-Barth and Weiler (2009), state that, to a structural engineer wind loading will not be the most critical case of consideration on a green roof.
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With regard to seismic loads, Gartner, 2008, state that when considering seismic loading, the entire saturated dead load of a green roof system is to be considered as part of the seismic mass.
2.15.2 Effects of structural elements
Different structural elements influence the success or failure of a green roof project in different ways. The following chapter identifies various elements and their associated effects on green roofs.
2.15.2.1 Roof type
In accordance with traditional building definitions, a roof can be considered as either being flat or sloped. The benefits of each vary. For instance, a flat roof will retain much more rain or snow, as compared to a sloped roof which will create a greater run-off. The choice of roof slope is often one of personal choice, practicality and purpose of the structure. Sloped roofs may have more of an aesthetic appeal for some, whereas flat roofs prove to be more practical in long spanning systems. Both roof types are subjected to high temperatures at direct exposure. However, this proves more severe on flat roofs due to direct exposure to sunlight at all times. With that said, according to Scholz-Barth and Weiler (2009), constructing and maintaining a green roof system is generally easier on a flat roof or a roof that has a slight slope. This comes as a result of a green roof system not being subjected to the gravity and shear forces that one would expect to act on a green roof system constructed on a sloped roof. However, Greenstone, et al. (2010), adds that flat roofs also carry a disadvantage i.e. if a roof is too flat it allows for water to accumulate which can lead to root rot and further damage to the plant life. Greenstone, et al. (2010), does not deny that green roof systems can be constructed on sloped roofs but goes on to state that on a sloped roof the pitch becomes critical. In cases where the pitch is in excess of 10°, the substrate material is subjected to the forces of gravity and causes the material to slump or slip off completely. The researchers go on to state that ideally a green roof system should lie on a roof that has a pitch of between 3°-10°, if a sloped roof is to be utilised.
2.15.2.2 Decking or structural slab
When a green roof system is designed and installed upon a structure, the structure’s roof is considered to be the floor that will serve as the primary support structure for the system.
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As such, this surface upon which the green roof will be constructed, spanning the length of the beams or joists, is the deck of the green roof system. The deck of a green roof system can be comprised of a variety of different material and structural systems, ranging from plywood to metal or concrete. Scholz-Barth & Weiler (2009), suggest that of these materials, reinforced concrete is the most suitable for use in green roof systems due to the large load-bearing capacity it can withstand. Based on the fact that a structural concrete deck may be cast in place with reinforcement or poured over a metal deck and made to fill it, once a suitable choice is made upon the deck or slab surface, this choice will then lead to the choice of a suitable waterproofing system. However, each choice will impact on the suitability of a green roof system. Materials such as tile, slate or metal roofs make installation of a green roof system and the functionality of waterproofing difficult. Of particular concern is the use of metal whose properties cause the expansion and contraction of the material as per fluctuation in temperature. These movements of the material cause the membranes within the green roof system to undergo stress. In addition, because metal is a good conductor of heat, when it is heated by radiant energy from the sun, the heat is transferred into the green roof system and directly into the vegetation and growing mediums. However, the temperature fluctuation effect can be negated through the use of thermal insulation.
Ensuring that retention of water within a green roof system is at an optimal level, is key to the survival of a green roof. Plants cannot utilise water if it is drained faster than they can absorb it and as a result runs the risk of dying out. On the contrary, if a green roof system gains too much water, the growing mediums can be faced with anaerobic conditions that will result in the soil becoming toxic to the vegetation.
Scholz-Barth & Weiler (2009), state that in order for both the roof deck and green roof system to drain any excess water, the gradient of the roof deck should be that of 1%. For concrete decks that are cast in-situ, a gradient of 2% should be applied to take into account the sag of the concrete over time.
2.15.2.3 Waterproofing
The main purpose of the waterproofing layer is to ensure that water (be it from rain, snow, or condensation) in the green roof system is kept out of the structure below. As previously discussed the choice of waterproofing should be coordinated with the other components within the green roof system to ensure the survival and long-term performance of the
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green roof. If the waterproofing layer fails, consequently it could result in the collapse of the green roof system.
In accordance with the findings of Gartner (2008), the following questions are to be raised in order to assist in determining drainage and water proofing:
1. What type of drainage plan will be in place or required?
2. What type of water proofing is to be provided?
3. Will there be any leak detection systems in place?
4. What type of drainage plan will be in place or required?
5. What type of water proofing is to be provided?
6. Will there be any leak detection systems in place?
2.15.3 Serviceability considerations 2.15.3.1 Deflection
Deflection criteria for green roof systems are generally calculated in the same way that one would perform on a normal roof structure. However, with green roof systems there are other criteria that must be taken into account when determining the most accurate deflection criteria. For instance, if membranes utilised for waterproofing will be susceptible to damage from deflection and/or ponding (Gartner, 2008).
2.15.4 Structural misconceptions
According to Gartner (2008), when working with green roofs most structural engineers live with three common misconceptions or misunderstandings that lead to over- conservative assumptions. Modern green roof systems aim to implement sustainable practices and often utilise a combination of engineered soils with lightweight insulation and drainage layers. As such, the first misconception made by some structural engineers is to utilise full saturated weight of soil for the entire depth of the green roof system. As a result, it becomes an over-conservative assumption due to the fact that a green roof system does not comprise entirely of soil. The second of these misconceptions, is the assumption that a green roof system is a soil load, as these generally cover lateral earth pressures, as opposed to a dead load.
2.15.6 Load combinations
When analysing envelopes, a structure should be assessed under two conditions. Firstly, the structure with its conventional roof and secondly, the structure with the green roof
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system. The envelope analysis strategy allows for the determination of maximum and minimum moment and shear conditions.
According to Gartner (2008), the following structural checks are relevant to green roof systems and should be taken into account, when applicable:
1. Verification of irregularities in seismic mass for conditions of conventional roof and fully saturated roof conditions.
2. Structural elements supporting green roofs i.e. gravity beams, seismic drags/collectors and any other connections, must be evaluated for conditions of high bending in combination with axial loads.
3. Careful evaluation is needed for punching shear in concrete slabs.
4. Careful evaluation of plastic hinges that are expected in lateral systems.
5. Consideration to the sequencing of construction of the shear wall and bracing, in an attempt to prevent dead loading